The instant invention relates to the use of a hybrid central receiver with a natural gas combined cycle power generating system. The system of the instant invention uses solar energy to supplement the operation of the natural gas combined cycle system by using the solar generated heat to preheat the combustion air. The solar generated heat is transferred to the natural gas cycle by an air/molten salt heat exchanger or air heater.
Combined cycle power plants which include a steam turbine to utilize waste heat from the gas turbine are well known. Typically such a plant includes a combustion turbine driving an electric generator, a heat recovery steam generator which receives exhaust gas from the combustion turbine, and a steam turbine driven by superheated steam for driving another electrical generator or for achieving other well known functions. The steam turbine is located in a closed loop connecting the inlet and outlet of the heat recovery steam generator. Such a combined cycle power plant is shown in U.S. Pat. No. 4,932,204 to Pavel et al.
The Pavel et al patent, however is not concerned with the use of a central receiver or the use of solar energy to preheat the air supplied to a combustion turbine. Rather, the patent adjusts the flow rate of feed water through an economizer section of a heat recovery steam generator to provide an excess flow which is used to preheat the fuel delivered to a combustion turbine.
Solar thermal central receiver power plants are also well known. Such power plants are large and currently costly to build, and thus such technology has not carried forth into development on a commercial scale. Designs for such plants have been known to use a central receiver which has the potential to be a very cost-effective solar technology for generating electricity. The most preferred well known central receiver design utilizes a tube-type receiver, molten salt as a heat transfer medium, and a steam turbine to convert the absorbed heat into electricity. Such a system is extremely expensive to build and will result in relatively high energy costs. The known system designs do not contemplate using a central receiver in a hybrid plant having a high-efficiency combined cycle (a gas turbine plus a steam turbine to utilize waste heat from the gas turbine).
A solar thermal power plant is also described in U.S. Pat. No. 4,167,856 to Seidel et al. This patent discloses an open air circuit wherein the effective heat obtained from solar energy is supplied to the air flow between a compressor and an expansion machine. This power plant utilizes a solar heater exposed directly to solar radiation. Thus the use of a heat transfer medium such as molten salt is not contemplated. Also, the reference is not directed to a combined cycle plant configuration nor does it recognize that such a combined cycle plant would result in cost savings when utilized with a central receiver. The reference does use the solar heater to directly heat air prior to combustion, but an additional air preheater is also required to preheat the air before it is directed into the compressor.
Another example of utilizing solar heat to preheat the air prior to combustion is shown in U.S. Pat. No. 4,259,836 to Finckh et al. As described by this patent, the solar energy function can be selectively disconnected from the gas turbine for conventional operation. Again the reference refers to direct preheating without the use of a central receiver and heat transfer medium. Also, the reference is not directed to a combined cycle power plant.
In the past, molten salt has been used as a heat transfer medium. For such a use the temperature of the molten salt is kept sufficiently high to keep the salt in its molten form, generally above 500.degree. F. Both U.S. Pat. No. 4,094,148 to Nelson, and U.S. Pat. No. 4,438,630 to Rowe disclose the use of molten salt for heat transfer purposes. Neither of the references however, discloses using molten salt to transfer heat from a solar central receiver. Nelson uses the salt to store heat released by exhaust gases derived from the combustion of gaseous products in a coal gasification process so that the energy can be utilized to generate increased electrical power during peak demand periods. The Rowe reference is directed to a co-generating steam supply system which utilizes a molten salt heat transfer fluid for utilizing the steam produced by the first unit for maintaining selected component operating temperatures in the second unit during periods when the second unit is not producing steam. The transporting of heat from a central receiver is not mentioned by either of the references.
Central receiver power systems are capable of generating large amounts of electrical energy. To provide such power, central receiver systems usually include an array of thousands of individually steered flat reflectors or mirrors and a central receiver on top of a tower. Such arrays are normally referred to as a heliostat field, while the individual reflectors are normally referred to as heliostats. The heliostat field redirects radiant solar energy to the receiver. The receiver collects and converts the radiant solar energy to thermal energy. Such a system is disclosed in U.S. Pat. No. 4,466,423 to Dolan et al.
The instant invention makes it possible to use a central receiver on a smaller scale than that contemplated for production solely through the use solar energy while still achieving enhanced energy production with associated cost savings.